Shadow-mask type color television tube with screening electrodes for converging the electron beams on the shadow mask

ABSTRACT

An electron gun assembly for use particularly in a shadow-mask color television cathode-ray tube has a plurality of (usually three) electron emissive areas adjacent which an accelerating electrode or electrodes is or are arranged for producing respective substantially collimated electron beams from electrons emitted by said areas. The collimated beams pass through an apertured screening electrode or electrodes so arranged that the space extending up to the electron emissive areas is effectively screened from electric fields in the tube beyond the screening electrode or electrodes.

United States Patent 11 13,617,790

[72] Inventor Douglas S. Hills [56] References Cited East Peekharn, Tonbridge, Kent, England UNITED STATES PATENTS [21] "f i 2 1969 2,887,598 5/1959 Benway 313 70 [221 e 2,957,106 /1960 Moodey 313/70 x Patented 1971 3,331,155 4/1968 Arnaud et al. 313/82 [73] Assgnee zfizgz gggfgg 3,448,316 6/1969 Yoshida et a1. 313/69 Priority p 1968 3,467,881 9/1969 Ohgoshlet al. 313/77 [33] Great Britain Primary Examiner-Roy Lake [31] 17,261/68 Assistant E.raminerV. Lafranchi Attorney-Holcombe, Wetherill & Brisebois [54] SHADOW-MASK TYPE COLOR TELEVISION ABSTRACT: An electron gun assembly for use particularly in SHADOW MASK a shadow-mask color television cathode-ray tube has a plurali- 15 Claim 7 D Fi ty of (usually three) electron emissive areas adjacent which an rawmg accelerating electrode or electrodes is or are arranged for [52] US. Cl 313/82, producing respective substantially collimated electron beams 313/85, 315/13 C from electrons emitted by said areas. The collimated beams [51] lnt.Cl ..H0lj 29/06, pass through an apertured screening electrode or electrodes HOlj 29/46, HOlj 29/56 so arranged that the space extending up to the electron emis- Field 01 Search 313/ C, sive areas is effectively screened from electric fields in the 69, 83, 82 BF, 85; 178/514 PD; 315/13 C, 31 tube beyond the screening electrode or electrodes.

SHADOW-MASK TYPE COLOR TELEVISION TUBE WITH SCREENING ELECTRODES FOR CONVERGING THE ELECTRON BEAMS ON THE SHADOW MASK This invention relates to electron gun assemblies for use in cathode-ray tubes and to cathode-ray tubes incorporating such gun assemblies. More particularly the invention concerns assemblies comprising a plurality of electron guns such as are used, for example, in color television cathode-ray tubes of the so-called shadow-mask type.

A shadow-mask cathode-ray tube has a screen which is provided with a large number of phosphor dots on its internal surface, that is, its surface facing the electron beam source. The phosphor dots are arranged in groups or triads of three, each triad comprising three different kinds of phosphor dot which upon excitation by a cooperating electron beam respectively emit the three primary colors, for example, red, blue and green, which together compose the picture to be displayed.

In order to produce the registration, three separate and distinct electron guns are provided at the end of the tube remote from the screen, each gun producing an independent electron beam for irradiating the respective cooperating phosphor of each triad.

In order to ensure correct registration of the phosphors in any triad only with the respective cooperating electron beam, a perforated conductive mask is situated close to the internal surface of the screen, and the three electron beams from the three guns are so directed that they can reach the phosphor triads only by passing through a particular perforation in the mask. The dimensions of the perforations in the mask are sure that each electron beam on passing through the perforation can impinge only upon the one phosphor dot of the triad, appropriate to the color to which that beam corresponds. Those portions of the electron beams which impinge upon the triads pass through the mask only for a small portion of the scanning period; during the remainder of the scanning period, the mask effectively shields the phosphor dots from the electron beams, and in so doing receives a considerable beam current which is conducted away through the mask.

in a conventional shadow-mask tube, and for reasons hereinafter recited, less than 20 percent of the beam current from each electron gun reaches the screen, the greater part of the remainder of the current being intercepted by the mask. In order, therefore, to achieve a display of adequate brightness the energy of the electrons must be sufficiently high to compensate for this loss. For example, in a shadow-mask tube operated at an extra high tension voltage of 27 Kilovolts each electron gun may produce a continuous beam current of 1 milliampere, rising to 3 milliamperes for short periods. This leads to a considerable power dissipation in the mask, resulting in heating of the mask. The heating may become sufficiently high to cause undesirable expansion of the mask, accompanied by distortion, loss of masking accuracy and consequent distortion of line balance in the picture displayed.

A conventional shadow-mask tube also suffers from the disadvantage that it is necessary, in view of the high voltage of operation, to make at least the screen and faceplate of the tube from nonpolarizing glass, which is much more expensive than the glass used in monochrome television cathode-ray tubes. One of the objects of the present invention is to provide an electron gun assembly, suitable for use with a cathode-ray tube of the shadow-mask type, in which very high working voltages are avoided, so that conventional glass can be used for cathode-ray tubes provided with the gun assembly.

The electron guns conventionally used in shadow-mask cathode-ray tubes are tetrode guns, having eihzel lens focusing. Such guns produce a beam which tends to defocus with increasing beam current, producing a spot on the screen of the cathode-ray tube which increases in size as the beam current increases. in a shadow-mask tube the maximum acceptable spot diameter is that which, when positioned symmetrically on one phosphor dot of a triad,just fails to overlap and excite adjacent dots. in practice the maximum spot diameter is typically about 0.03 in. at full beam current. Now the phosphor dot diameter is usually of the order of one-half of the beam diameter, i.e. 0.015 in. so that only one-quarter of the total cross-sectional area of the electron beam is effective in activating a dot at full beam current.

Furthermore, the electron beam emitted by a tetrode gun has a Gaussian intensity distribution across a diameter. Assuming that the beams at maximum current infringes symmetrically on the spot, the beam intensity, and, therefore, the spot luminance, at the edges ofthe spot will be less than 50 percent of that at the center of the spot.

Accordingly in order to produce an acceptable level of excitation over the whole of a dot in any triad, only the central position of the beam can be utilized, and the total beam area which is generated by the electron gun, must be considerably greater than the aperture in the mask through which it passes. The shadow-mask accordingly absorbs not only the energy of the electron beam during the scanning periods, when the mask prevents registration, but also during registration, absorbs the surplus energy in these parts of the beams which must be generated, but which cannot be used. This power dissipation in the mask produces further loss of masking accuracy and line distortion.

According to one aspect of the present invention, an electron gun assembly for use in a cathode-ray tube comprises a plurality of electron emissive areas adjacent which accelerating means are arranged for producing respective substantially collimated electron beams from electrons emitted by respective said areas, together with screening means through which the collimated beams may pass and which are effective to screen the space extending up to the electron emissive areas from electric fields beyond the screening means.

Preferably the electron emissive areas are provided on coplanar surfaces and the accelerating means comprise an electrode arrangement capable of producing in the space adjacent the cathode surfaces an electric field having unipotential surfaces parallel to said surfaces.

The expression unipotential surface" is understood to mean a surface over which there are not potential differences. Preferably the said unipotential surfaces and the electron emissive areas are plane parallel surfaces, and in one preferred embodiment of the invention the electron emissive areas comprise respective areas of a common flat cathode surface. Alternatively, the electron emissive areas may be provided on respective separate cathodes.

Electrons emitted from the respective emissive areas will tend to cross the unipotential surfaces of the accelerating electric field normally. The present invention accordingly ensures that substantially all the emitted electrons ultimately travel along paths perpendicular to the emissive areas, producing a plurality of beams (usually three in number) in which lateral velocity components are reduced to a minimum. Subsequent focusing and scanning of the beams can, therefore, be effected with a minimum energy input.

Moreover, since each electron beam produced by the gun assembly originates from a sharply defined area and contains substantially no lateral energy components, the cross section of each beam has a substantially uniform intensity distribution and a sharply defined periphery. When using this electron gun assembly in a shadow-mask cathode-ray tube each phosphor dot can, therefore, be excited by the respective electron beam to a uniform luminance level. Moreover, a greater proportion of the beam current can pass through the shadow-mask to the phosphor screen, so that the current flowing in the mask is reduced.

The electrode arrangement preferably includes a first accelerating electrode parallel to the coplanar surfaces of the electron emissive areas and disposed between said surfaces and the screening means, said first electrode having respective apertures through which the respective collimated electron beams may pass.

According to one embodiment of the invention the electrode arrangement may have a Pierce configuration and may include an outwardly flared electrode surrounding emissive areas, the said flared electrode being concave towards the screening means.

The screening means preferably comprise an electrode having relatively deep apertures to permit passage of the respective electron beams, the ratio of the diameter of each aperture to the thickness of the electrode being such as to produce effective screening. Alternatively, the screening means may comprise an electrode having respective apertures which permit the passage of the respective electron beams and which are covered by conductive wire mesh effective to screen the apertures. In a further alternative construction the screening means comprise two thin electrodes having respective pairs of aligned apertures therein and spaced apart by a distance not less than the diameters of said apertures.

Respective control electrodes having respective control leads connected thereto may be provided to control the relative intensities of the respective electron beams independently. When the gun assembly is installed in a shadow-mask color television cathode-ray tube the relative potentials applied to the control electrodes are derived from a television chrominance signal. In one embodiment of the invention the control electrodes comprise respective areas insulated from each other, of a common electrode member, each area having an aperture therein through which the respective electron beam may pass. Said control electrodes may form at least part of the screening means.

Further accelerating means comprising at least one apertured electrode are preferably disposed beyond the screening means to further accelerate the electron beams which pass through the screening means. Said further accelerating means in a preferred embodiment of the invention comprise a plurality of plane parallel electrodes having respective aligned apertures therein through which the respective electron beams may pass.

The invention also includes a shadow-mask color television cathode-ray tube having an electron gun assembly as herein defined effective to produce three electron beams for exciting phosphor dots of three respective primary colors. An additional beam may be included for registration purposes in the case where a shadow-mask is not used.

The invention will be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic longitudinal sectional view of a shadow-m ask cathode-ray tube provided with an electron gun assembly according to the invention;

FIG. 2 is a diagrammatic exploded view of part of an electron gun assembly according to one embodiment of the invention;

FIG. 3 is a diagrammatic view of a control electrode forming part of an electron gun assembly according to a preferred embodiment of the invention;

FIG. 4 shows an electro magnetic focusing arrangement forming a further part of the electron gun assembly in one embodiment thereof;

FIG. 5 shows an electrostatic focusing arrangement forming a further part of the electron gun assembly in another embodiment thereof;

FIG. 6 illustrates diagrammatically part of an electron gun according to a modification of the embodiment of FIG. 2, and

FIG. 7 illustrates diagrammatically part of an electron gun according to another modification of the embodiment of FIG. 2.

Referring first to FIG. 1, the cathode ray tube shown diagrammatically is of the shadow-mask type used in a color television received in the so-called National Television Systems Committee system. The tube comprises an evacuated electrically insulating envelope 2, usually of glass, having at one end a light transmitting faceplate 3, and at the other end a neck 4 within which an electron gun assembly Sis disposed.

A phosphor screen 6 is provided within the envelope 2 close to the internal surface of the faceplate 3, or alternatively formed directly on this internal surface. As known, the screen 6 consists of a large number of groups or triads of circular phosphor dots of a uniform diameter (typically 0.0l5 in. arranged over the entire area of the screen 6. Each triad comprises three phosphor dots of three different primary colors, for example, red, green, and blue, the centers of the dots being disposed at the vertices of an equilateral triangle, and separated by distances equal to the diameter of a phosphor dot.

Closely spaced from and parallel to the phosphor dot screen 6 is a shadow-mask 7 comprising a rigid electrically conductive plate having a regular array of perforations therein, each perforation being associated with a different respective phosphor dot triad of the screen 6.

The electron gun assembly 5 provides a plurality of electron beams, in this case three beams indicated diagrammatically at bl, b2, b3, the assembly 5 including means, as hereinafter described, for causing the three beams to converge at the shadow-mask 7, the respective beams b1, b2, b3 being so directed in relation to the positions of the phosphor dots that, when the beams converge in a perforation of the mask 7, they impinge on different respective phosphor dots of the triad associated with that perforation. In this way the beams b1, b2, b3 can be arranged to excite the three respective primary colors on the screen 6.

Deflection coils, shown diagrammatically in broken lines at 8, are arranged externally of envelope 2 and are energized with repetitive scanning waveforms in the well-known manner to cause the beams b1, b2, b3 to execute a repetitive roster scan of the entire area of the screen 6, the scanning path being such that the intersection point of the three beams bl, b2, b3 passes successively over each of the perforations of the shadow-mask 7. By controlling the relative intensities of the three beams in synchronism with each scan the relative intensities of the three primary colors excited at each of the triads on the screen 6 can be controlled so as to build up a color picture.

Part of the electron gun assembly 5 is shown in the exploded view of FIG. 2. The three electron beams bl, b2 and b3 are produced by three respective cathodes 10,, 10,, 10;, having respective coplanar electron emissive surfaces 11,, 11 and 11 may comprise three separate areas on a plane surface ofa common cathode having a common heater.

A first accelerating electrode 12 is spaced a short distance (e.g. 0.004 in. from the emissive surfaces 11, the electrode 12 comprising a flat metal plate about 0.005 in. thick, having three apertures 13,, 13 13 therein aligned with the respec tive emissive surfaces 11,, 11 11 This electrode 12 has a connection 12' to a circuit point (not shown) externally of the tube envelope and is positively biased to a potential which exceeds the common potential of the cathodes 10 by about 5 volts.

Alternatively, the accelerating electrode 12 may be spaced further from the emissive surfaces 11 and an outwardly flared or dished electrode 12' (FIG. 6), may surround the cathodes 10, the dished electrode and the electrode 12 together forming a Pierce electrode arrangement (see, for example, .I. R. Pierce, Theory & Design of Electron Beams, Van Nostrand, 1964).

With either of these arrangements an electron accelerating field is produced which, at least in the vicinity of the electrode 12, has plane parallel unipotential surfaces parallel to the electrode I2, and, therefore, to the electron emissive surfaces. Electrons emanating from the three emissive surfaces 11,, 11 11 cross these unipotential surfaces normally, so that the three beams bl, b2, b3, passing through the apertures 13,, 13 13 are collimated and parallel to the axis of the electron gun assembly 5. Moreover, the electric field at each of the emissive surfaces 11,, 11 11;, is substantially uniform, so that the resulting beams bl, b2, b3 have substantially uniform current densities across their widths. A screening and control electrode 14 is passed from and parallel to the accelerating electrode 12, the electrode 14 having respective apertures 15,, 15 15;, for the passage of the respective electron beams b1, b2, b3. The electrode 14 comprises a fiat metal plate having a thickness in relation to the diameters of the apertures 15 such as to ensure electrostatic screening of the space between the electrode 14 and the cathodes from electric fields produced by further accelerating electrodes 16, provided on the side of the electrode 14 remote from the cathodes l0. Distortion of the unipotential surfaces in the electron emission system is thereby prevented. In one embodiment the electrode 14 may be 0.030 in. thick and have apertures 0.020 in. in diameter. Alternatively, the screening electrode 14 may comprise two thin plates 14a, 14b (FIG. 7) having respective pairs of aligned apertures therein and spaced apart by a distance not less than the diameters of said apertures. Modulation of the respective electron beams bl, b2, b3 in accordance with three respective chrominance signal components is effected by means of respective control grids 17,, 17 17 insulated from each other and disposed at each respective aperture 15,, 15 15 of the screening electrode 14, each grid 17 having connected thereto a respective control lead 18 which passes outwardly through the envelope of the tube (not shown in FIG. 2) and to which a potential derived from a respective chrominance signal components is applied.

Where the modulation applied to the grids, 17,, 17 17 is substantial, the electrode 12 is arranged to effect further screening of the space between the electrode 12 and the emissive surfaces 11. For this purpose the apertures 13 in the electrode 12 should have a penetration factor for electric fields which is less than 70 percent for example by suitably selecting the diameter to depth ratio of the apertures 13.

Screening may alternatively be effected at the electrode 14 and, if necessary, at the electrode I2, by covering each aperture in the electrode by a conductive wire mesh 14a (FIG. 6).

An alternative form of control electrode for effecting the chrominance modulation of the three electron beams is shown diagrammatically in FIG. 3. An electrode member comprising a circular plate 14 is divided into three equal sector-shaped areas, 14a, 14b, 14, each of which is insulated from the others and constitutes a respective control electrode for the respective beams bl, b2, b3. Each area is provided with a respective aperture 15a, 15b, 15c, through which the respective beam b1, b2, b3 passes, and with a control lead 18a, 18b, 180, by which the respective control potential derived from the chrominance signal is applied to the respective electrode. The plate 14' is arranged to have a field penetration factor such that it effects shielding of the space extending up to the electron emissive surface 11.

The further accelerating electrodes 16 comprise respective plane parallel metal plates spaced along the axis of the gun assembly by for example, 0.040 in. and each having respective apertures 19,, 19 19 therein for the passage of the respective electron beams b1, b2, b3. Eleven electrodes 16 are provided, but for simplicity of illustration, only two of these electrodes are shown in FIG. 2. At least some of the electrodes 16 closer to the screening electrode 14 are connected to the latter and in operation of the gun are maintained at a positive potential of the order of 1,000 volts, while the remaining electrodes 16 are interconnected electrically, and are maintained at a positive potential of the order E.I'I.T./20 volts.

The further accelerating means constituted by the electrodes 16 has associated therewith a electromagnetic (FIG. 4) or electrostatic (FIG. 5) system for focusing each beam and for causing the beams bl, b2, b3 to converge at the shadowmask 7.

In FIG. 4 the final electrode 16,, of the further accelerating means is shown. Aligned with each respective aperture 19,, I9 19,, in the electrode 16,, are respective pairs of magnetic beam deflection yokes 20,, 21,; 20 21 20 21 through which the respective beams bl, b2, b3 pass successively. The yokes 20,, 20 20, are arranged to cause beam deflections in one direction, for example, horizontally in the FIG., while the yokes 21 21 21 are arranged to cause deflections in a perpendicular direction, for example, vertically. The deflection yokes have magnetic coils (not shown) which are connected electrically to the beam deflection circuit of the cathode-ray tube in such a manner as to effect dynamic convergence of the beams b1, b2, b3. That is to say, small beam deflecting adjustments are made by the yokes 20, 21 in accordance with the signals applied to the deflection coils 8 (that is, in accordance with the part of the screen 6 irradiated by the electron beams) to adjust automatically the convergence of the beams bl, b2, b3, so that the point of convergence of the beams always coincides with the shadow-mask 7.

FIG. 5 shows an alternative focusing and converging arrangement. In this case, the convergence-adjusting deflections are made electrostatically by respective pairs of deflector plates 22,, 22 22 and 23,, 23, associated with the respective apertures 19,, 19 19 of the final two accelerating electrodes 16a, 16b respectively. In the illustrated arrangements, the plates 22 effect vertical components of deflection while the plates 23 effect horizontal components of deflection. As in the embodiment of FIG. 4, dynamic convergence of the beams bl, b2, and b3 is effected by controlling the voltages applied to the respective plates 22, 23 in accordance with the beam deflections effected by the deflection coils 8.

The three beams b1, b2, b3 produced by the electron gun assembly of the invention are accurately collimated and, moreover, have a current density distribution over the beam cross section which is substantially uniform, falling sharply to zero at the beam edges, the cross-sectional profiles of the beams being determined by the shape of the apertures 13, 15 and 19 in the respective electrodes l2, l4 and 16. Since a greater proportion of each beam bl, b2, b3 passes through the shadow-mask 7 to impinge on the screen 6, the power consumption in cathode-ray tubes employing electron gun assemblies according to the invention is less for a given picture brightness than in conventional shadow-mask tubes.

In an alternative embodiment of the invention, the electron gun assembly may be arranged to produce four electron beams. In this case, three of the four beams are used as the chrominance beam to excite the respective chrominance phosphor dots in the triads, the fourth beam, being used for registration purposes to determine in any one triad the coincidence ofthe chrominance beam with the respective dots.

At registration, the fourth beam may, for example, activate suitably disposed phosphors emitting ultraviolet radiation which can be detected by any suitable means, for example, a photoelectric multiplier. Such a device would produce an electric output signal only when the chrominance beams were in positions of coincidence and would be arranged automatically to cut off the chrominance beams in noncoincidence positions. In this arrangement, the shadow-mask would be superfluous and could be dispensed with.

I claim:

1. An electron gun assembly for use in a cathode-ray tube, comprising in combination a plurality of coplanar electron emissive areas,

accelerating electrode means adjacent said emissive areas and having surfaces parallel thereto,

screening means adjacent said accelerating electrode means on the side remote from said electron emissive areas said screening means electrostatically screening the space extending up to said electron emissive areas from electric fields beyond said screening means,

means connecting said accelerating electrode means to a source of electrical potential thereby producing in the space between said screening means and said coplanar electron emissive areas an electric field having unipotential surfaces parallel to said electron emissive areas whereby respective substantially collimated electron beams each with a substantially uniform electron distribution across its width are formed from electrons emitted by respective said areas, and

respective control electrodes and respective control leads connected to said control electrodes, said electrodes controlling the relative intensities of said respective electron beams independently in operation of said gun assembly.

2, The electron gun assembly of claim 1 wherein said electron emissive areas comprise respective areas of a common flat cathode surface.

3. The electron gun assembly of claim 1 wherein said respective electron emissive areas are each provided by a separate cathode.

4. The electron gun assembly of claim 1 wherein said accelerating electrode means includes an outwardly flared electrode surrounding said emissive areas, said flared electrode being concave towards said screening means.

5. The electron gun assembly of claim 1 wherein said accelerating electrode means includes a first accelerating electrode parallel to said coplanar surfaces of said electron emissive areas and disposed between said surfaces and said screening means, and means defining in said first electrode respective apertures through which respective collimated electron beams pass in operation ofsaid gun assembly.

6. The electron gun assembly of claim 1 wherein said screening means comprise a screening electrode and means defining in said screening electrode respective apertures permitting passage of respective electron beams, the ratio of the diameter of each said aperture to the thickness of said screening electrode being sufficient to produce effective screening.

7. The electron gun assembly of claim 1 wherein said screening means comprise a screening electrode, means defining in said screening electrode respective apertures permitting the passage of respective electron beams, and conductive wire mesh covering and screening each said aperture.

8. The electron gun assembly of claim 1 wherein said screening means comprise two thin apertured electrodes, said respective electron beams passing through respective pairs of aligned apertures in said electrodes, and said electrodes being spaced apart by a distance not less than the diameters of said apertures.

9. The electron gun assembly of claim 1 wherein said control electrodes comprise respective portions, insulated from each other, ofa common electrode member, and means defining in each said portion a respective aperture through which said respective electron beam passes in operation of said gun assembly.

10. The electron gun assembly of claim 1 wherein said control electrodes form at least part of said screening means.

11. The electron gun assembly of claim 1 including further accelerating means comprising at least one apertured electrode disposed beyond the screening means and effective to further accelerate said electron beams which pass through said screening means.

12. The electron gun assembly of claim 11, wherein said further accelerating means comprise a plurality of plane parallel electrodes and means defining in said electrodes respective aligned apertures through which said respective electron beams pass in operation of said gun assembly.

13. A shadow-mask color television cathode-ray tube comprising an evacuated envelope,

a screen and a shadow-mask at one end of said envelope,

an electron gun assembly at the other end of said envelope,

said electron gun assembly including three coplanar electron emissive areas,

accelerating electrode means adjacent said emissive areas,

screening means adjacent said accelerating electrode means on the side remote from said electron emissive areas said screening means screening the space extending up to said electron emissive areas from electric fields beyond said screening means,

means connecting said accelerating electrode means to a source of electrical potential thereby producing, in the space between said screening means and said coplanar electron emissive areas, an electric field having unipotential surfaces parallel to said emissive areas whereby respective substantially collimated electron beams each having a substantially uniform electron distribution across its width are produced from electrons emitted by respective said areas, respective control electrodes and respective control leads connected to said control electrodes, said electrodes controlling the relative intensities of said respective electron beams independently in operation of said gun assembly, and means for maintaining the point of convergence of said beams on said shadow-mask for all positions of said beams.

14. The shadow-mask color television cathode-ray tube of claim 12, including electromagnetic beam deflector elements provided on said gun assembly for effecting deflection of each electron beam to adjust the convergence of said beams automatically in dependence on the deflection of said beams, thereby to maintain the point of convergence ofsaid beams on said shadow-mask for all positions of said beams.

15. The shadow-mask color television cathode-ray tube of claim 13, including electrostatic beam deflector elements pro vided on said gun assembly for effecting deflection of each electron beam to adjust the convergence of said beams auto matically in dependence on the deflection of said beams, thereby to maintain the point of convergence of said beams on the shadow-mask for all positions ofsaid beams.

* a a a 

1. An electron gun assembly for use in a cathode-ray tube, comprising in combination a plurality of coplanar electron emissive areas, accelerating electrode means adjacent said emissive areas and having surfaces parallel thereto, screening means adjacent said accelerating electrode means on the side remote from said electron emissive areas said screening means electrostatically screening the space extending up to said electron emissive areas from electric fields beyond said screening means, means connecting said accelerating electrode means to a source of electrical potential thereby producing in the space between said screening means and said coplanar electron emissive areas an electric field having unipotential surfaces parallel to said electron emissive areas whereby respective substantially collimated electron beams each with a substantially uniform electron distribution across its width are formed from electrons emitted by respective said areas, and respective control electrodes and respective control leads connected to said control electrodes, said electrodes controlling the relative intensities of said respective electron beams independently in operation of said gun assembly.
 2. The electron gun assembly of claim 1 wherein said electron emissive areas comprise respective areas of a common flat cathode surface.
 3. The electron gun assembly of claim 1 wherein said respective electron emissive areas are each provided by a separate cathode.
 4. The electron gun assembly of claim 1 wherein said accelerating electrode means includes an outwardly flared electrode surrounding said emissive areas, said flared electrode being concave towards said screening means.
 5. The electron gun assembly of claim 1 wherein said accelerating electrode means includes a first accelerating electrode parallel to said coplanar surfaces of said electron emissive areas and disposed between said surfaces and said screening means, and means Defining in said first electrode respective apertures through which respective collimated electron beams pass in operation of said gun assembly.
 6. The electron gun assembly of claim 1 wherein said screening means comprise a screening electrode and means defining in said screening electrode respective apertures permitting passage of respective electron beams, the ratio of the diameter of each said aperture to the thickness of said screening electrode being sufficient to produce effective screening.
 7. The electron gun assembly of claim 1 wherein said screening means comprise a screening electrode, means defining in said screening electrode respective apertures permitting the passage of respective electron beams, and conductive wire mesh covering and screening each said aperture.
 8. The electron gun assembly of claim 1 wherein said screening means comprise two thin apertured electrodes, said respective electron beams passing through respective pairs of aligned apertures in said electrodes, and said electrodes being spaced apart by a distance not less than the diameters of said apertures.
 9. The electron gun assembly of claim 1 wherein said control electrodes comprise respective portions, insulated from each other, of a common electrode member, and means defining in each said portion a respective aperture through which said respective electron beam passes in operation of said gun assembly.
 10. The electron gun assembly of claim 1 wherein said control electrodes form at least part of said screening means.
 11. The electron gun assembly of claim 1 including further accelerating means comprising at least one apertured electrode disposed beyond the screening means and effective to further accelerate said electron beams which pass through said screening means.
 12. The electron gun assembly of claim 11, wherein said further accelerating means comprise a plurality of plane parallel electrodes and means defining in said electrodes respective aligned apertures through which said respective electron beams pass in operation of said gun assembly.
 13. A shadow-mask color television cathode-ray tube comprising an evacuated envelope, a screen and a shadow-mask at one end of said envelope, an electron gun assembly at the other end of said envelope, said electron gun assembly including three coplanar electron emissive areas, accelerating electrode means adjacent said emissive areas, screening means adjacent said accelerating electrode means on the side remote from said electron emissive areas said screening means screening the space extending up to said electron emissive areas from electric fields beyond said screening means, means connecting said accelerating electrode means to a source of electrical potential thereby producing, in the space between said screening means and said coplanar electron emissive areas, an electric field having unipotential surfaces parallel to said emissive areas whereby respective substantially collimated electron beams each having a substantially uniform electron distribution across its width are produced from electrons emitted by respective said areas, respective control electrodes and respective control leads connected to said control electrodes, said electrodes controlling the relative intensities of said respective electron beams independently in operation of said gun assembly, and means for maintaining the point of convergence of said beams on said shadow-mask for all positions of said beams.
 14. The shadow-mask color television cathode-ray tube of claim 12, including electromagnetic beam deflector elements provided on said gun assembly for effecting deflection of each electron beam to adjust the convergence of said beams automatically in dependence on the deflection of said beams, thereby to maintain the point of convergence of said beams on said shadow-mask for all positions of said beams.
 15. The shadow-mask color television cathode-ray tube of claim 13, including electrostatic beam deflector elEments provided on said gun assembly for effecting deflection of each electron beam to adjust the convergence of said beams automatically in dependence on the deflection of said beams, thereby to maintain the point of convergence of said beams on the shadow-mask for all positions of said beams. 